Vacuum casting apparatus and method using the same

ABSTRACT

A vacuum casting apparatus of the invention in which a melt is introduced into a mold cavity under vacuum, comprises (a) a vacuum vessel having at least one opening at its bottom; (b) a mold disposed within the vacuum vessel and having a mold cavity, a runner having an opening at a position under the opening of the vacuum vessel and extending along at least partially the side of the mold cavity, the mold cavity communicating with the runner through a plurality of filling passages, and a suction recess formed near a riser; and (c) a vacuum means communicating with the vacuum vessel. When the vacuum means evacuates the interior of the vacuum vessel, the mold cavity is rapidly evacuated to result in rapid filling of the cavity with a melt.

TECHNICAL FIELD

The present invention relates to a vacuum casting apparatus and a vacuumcasting method using the apparatus. More particularly, the presentinvention relates to an apparatus and a method for casting articles ofpoor castability such as complicated-shaped or thin stainless steelcasting or heat-resistant cast steel, etc.

BACKGROUND ART

In the production of a thin cast article having a thin portion of 5 mmthick or less, the fluidity of a melt introduced into a mold cavity israpidly decreased because a part of the melt is rapidly cooled andeasily solidified upon coming into contact with the internal wall of themold cavity. This results in defects such as insufficient filling of themold cavity, etc. In the production of a cast article of complicatedshape, air and gases generated from the mold material are likely to beintroduced into the resulting cast articles as defects such as blowholes. Thus, a defectless cast article which is thin and has complicatedshape is difficult to be produced.

As a method of producing a thin cast article having complicated shape, alost wax process has been known. In this method, a ceramic mold isheated to 700°-900° C. prior to the filling of the cavity with a melt toretard the cooling of the melt introduced into the cavity, therebykeeping the melt highly flowable. However, since a ceramic mold isexpensive, the production cost of a thin cast article having acomplicated shape would be extremely high.

As an alternative method, JP-A-60-56439 discloses a gypsum mold providedwith a cavity, runner, etc., in which a refractory filter having a gaspermeability higher than that of the gypsum is disposed in an arearanging from the neighborhood of a last-filled portion of the cavity tothe outside surface of the gypsum mold, thereby enhancing the evacuationcapability to increase the fluidity of the melt and prevent the defectdue to gas. The gypsum mold is produced by hydration-setting a gypsumslurry and drying the hardened gypsum. This method utilizing the gypsummold, as is the case of the lost wax process mentioned above, has beenknown as one of the precision casting methods for producing a castarticle of a high dimensional accuracy, and has been used for producingdies, parts for machines, artistic handicrafts, etc.

However, since the production of a gypsum mold includes the steps ofkneading, pouring, hydralion setting, pattern draw, drying, etc. takes along period of time, over 48 hours, the productivity of this method ispoor. Further, since the gas permeability of the gypsum mold isextremely low, it leads to difficulties in determining the castingdesign for evacuation and pressurization at filling of a cavity with amelt. In addition, since the cooling rate of a gypsum mold is low, themelt in the mold solidifies extremely slowly. Therefore, in the case ofcasting a thin article of complicated shape, a shrinking defect islikely to occur, resulting in a low yield of the desired cast article.

Recently, a vacuum casting method as disclosed in JP-B-60-35227, etc.has come to be used. In this method, a melt is introduced into a moldcavity by evacuating a mold. However, in this method, air is likely tobe incorporated into the melt through a mold portion which is notimmersed in the melt, failing to obtain a sufficient vacuum. Further,although it is applicable to casting of articles of low height andsimple shape, it is difficult to be applied to casting of high and thickarticles of complicated shape.

JP-A-64-53759 discloses an apparatus in which a mold provided with arunner passing through the mold is disposed in a vacuum vessel, theupper end of the runner being closed with a stopper which does not passa melt through it. A mold cavity, runner, etc. is filled with a melt byadjusting the pressure applied on the upper end of the runner passingthrough the mold lower than the pressure of the interior of the vacuumvessel surrounding the mold. However, in this art, since the vessel isevacuated through a hole positioned above the sprue, a sufficient vacuumcan not be achieved at last-filling portions of the mold cavity, riser,run-off, etc.

JP-A-2-303649 discloses a vacuum casting method in which a mold havingrammed particulate matter around it is maintained in a vacuum vessel byvirtue of vacuum, and the mold is immersed into a melt therebyintroducing a melt into the mold. In this method, however, since a moldis immersed in a melt together with the rammed particulate matter aroundit, the melt is disturbed before and after the immersion of the mold tocause an incorporation of air into the melt. Further, since a mold andrammed particulate matter around it project out from the vacuum vessel,air is likely to be incorporated into the melt from the bottom portionof the mold.

As mentioned above, the filling of a mold cavity with a melt isinsufficient in the prior art method. In particular, the casting of athin cast article, 5 mm thick or less, particularly 3 mm thick or less,having a complicated shape is difficult in the prior art method.

Accordingly, an object of the present invention is to provide a vacuumcasting apparatus suitable for producing a cast article, particularly athin cast article having a complicated shape, without suffering fromcasting defects such as insufficient filling, blow holes, etc. in a goodproductivity.

Another object of the present invention is to provide a vacuum castingmethod which shows the effect mentioned above.

DISCLOSURE OF INVENTION

As a result of intense research in view of the above object, theinventors have found that a remarkably high suction effect can beobtained by providing a suction recess near a mold cavity, a riser and arun-off of a mold disposed in a vacuum vessel, and found that a feedingeffect can be remarkably enhanced by communicating a mold cavity and arunner via at least two filling passages, thereby enabling to produce ahigh-quality, thin cast article having a complicated shape in a low costand a good productivity. The present invention has been accomplishedbased on these findings.

Thus, a vacuum casting apparatus according to a first embodiment of thepresent invention comprises:

(a) a vacuum vessel having at least one opening at the bottom thereof;

(b) a mold disposed in the vacuum vessel and having a runner and a moldcavity communicating with the runner, the runner having an opening atthe opening of the vacuum vessel; and

(c) a vacuum means communicating with the vacuum vessel; wherein asuction recess having an opening on the top surface of the mold isdisposed in the vicinity of a portion of the mold cavity which is mostdistant from the opening of the runner and which is lastly filled with amelt of casting material, and wherein the suction recess is so disposedthat a distance between the bottom of the suction recess and the portionof the mold cavity is smaller than a distance between the outer surfaceof the mold and any other portions of the mold cavity, thereby rapidlyfilling the mold cavity with the melt.

A vacuum casting method according to a first embodiment of the presentinvention comprises the steps of:

(a) disposing a mold having a runner and a mold cavity communicatingwith the runner in a vacuum vessel having at least one opening at thebottom thereof so that an opening of the runner is positioned in an areaof the opening of the vacuum vessel;

(b) disposing a suction recess having an opening on the top surface ofthe mold in a vicinity of a portion of the mold cavity which is mostdistant from the opening of the runner and which is lastly filled with amelt of casting material so that a distance between the bottom of thesuction recess and the portion of the mold cavity is smaller than adistance between the outer surface of the mold and any other portions ofthe mold cavity; and

(c) evacuating the mold by operating a vacuum means connected to thevacuum vessel thereby rapidly filling the mold cavity with the melt.

A vacuum casting apparatus according to second embodiment of the presentinvention comprises:

(a) a vacuum vessel having at least one opening at the bottom thereof;

(b) a mold disposed within the vacuum vessel and having a mold cavityformed therein, a runner having an opening under the opening of thevacuum vessel and extending along at least partially the side of themold cavity, the mold cavity communicating with the runner through aplurality of filling passages, and a suction recess having an opening onthe top surface of the mold and disposed in a vicinity of a portion ofthe mold cavity which is most distant from the opening of the runner andwhich is lastly filled with a melt of casting material; and

(c) a vacuum means communicating with the vacuum vessel; whereby themold cavity is evacuated through the suction recess by operating thevacuum means more rapidly than mold portions other than the mold cavityto rapidly fill the mold cavity with the melt.

A vacuum casting method according to second embodiment of the presentinvention comprises the steps of:

(a) disposing in a vacuum vessel having at least one opening at thebottom thereof a mold having a mold cavity formed therein and a runnerextending along at least partially the side of the mold cavity, the moldcavity communicating with the runner through a plurality of fillingpassages, so that an opening of the runner is positioned under theopening of the vacuum vessel;

(b) disposing a suction recess having an opening on the top surface ofthe mold in a vicinity of a portion of the mold cavity which is mostdistant from the opening of the runner and which is lastly filled with amelt of casting material so that a distance between the bottom of thesuction recess and the portion of the mold cavity is smaller than adistance between the outer surface of the mold and any other portions ofthe mold cavity; and

(c) evacuating the mold by operating a vacuum means connected to thevacuum vessel thereby rapidly filling the mold cavity with the melt.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view showing a vacuum castingapparatus according to the first embodiment of the present invention;

FIG. 2 is a schematic cross sectional view showing a modified embodimentof the vacuum casting apparatus shown in FIG. 1;

FIG. 3 is a schematic cross sectional view showing another modifiedembodiment of the vacuum casting apparatus shown in FIG. 1;

FIG. 4 is a schematic cross sectional view showing a vacuum castingapparatus according to the second embodiment of the present invention;

FIG. 5 is a schematic cross sectional view showing a modified embodimentof the vacuum casting apparatus shown in FIG. 4;

FIG. 6 is a schematic cross sectional view showing another modifiedembodiment of the vacuum casting apparatus shown in FIG. 4;

FIG. 7 is a cross sectional view taken along the line A--A of FIG. 6;

FIG. 8 is an illustration showing the filling condition of a cavityobtained by a measurement and a computer simulation; and

FIG. 9 is a graph showing vacuum degrees of some portions of the vacuumcasting apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail.

1! Cast Steel

The vacuum casting apparatus and vacuum casting method of the presentinvention is preferably used to cast a melt of steel, which has a hightemperature and is difficult to be cast into a thin cast article. A caststeel produced by the vacuum casting apparatus and vacuum casting methodhas a high heat resistance and a high oxidation resistance. Thecomposition of such a cast steel is, for example, as follows:

C: 0.05-0.45 weight %,

Si: 0.4-2 weight %,

Mn: 0.3-1 weight %,

Cr: 16-25 weight %,

W: 0-3 weight %,

Ni: 0-2 weight %,

Nb and/or V: 0.01-1 weight %, and

Fe and inevitable impurities: balance.

A cast steel having the above composition has, in addition to a usualα-phase, a so-called α'-phase (a-phase+carbides) transformed fromλ-phase. The area ratio of α'-phase is preferred to be 20-90% based onthe combined area of α-phase and α'-phase.

The vacuum casting apparatus and vacuum casting method of the presentinvention will be described in detail with reference to the drawings.

2! First embodiment

FIG. 1 is a schematic cross sectional view showing a vacuum castingapparatus according to a first embodiment of the present invention.

In FIG. 1, the vacuum casting apparatus 1 has a vacuum vessel having atleast one opening at its bottom, and a mold having a mold cavity,runner, etc. and disposed in the vacuum vessel. The vacuum vessel isevacuated from the upper side thereof to suck a melt from a runner at alower end of the mold thereby filling a mold cavity. Specifically, thevacuum casting apparatus 1 has a vacuum vessel 2 (a iron vacuum vesselhaving a 600 mm inner diameter and 800 mm height, for example) which hasan opening 3 at the bottom thereof. The top of the vacuum vessel 2 ishermetically closed with a cover 2a. The cover 2a is provided with aflexible tube 9 which is connected to a vacuum means 11 such as a vacuumpump, etc. via a vacuum regulating means 10.

The vacuum vessel 2 has a sand mold 4 incorporated therein. In thepresent invention, a sand mold made of silica sand, etc. is preferablein view of the castability and gas permeability. For example, a splitsand mold consisting of two vertical sections, which is molded by coldbox process, is preferred. The sand mold 4 has at its lower end anentrance portion 5 projecting downward and is disposed in the vacuumvessel 2 so that the entrance portion 5 projects downward from theopening 3.

In the sand mold 4, a runner 6 having, for example, a cross-section of10 mm long and 10 mm wide, extends vertically from the bottom of theentrance portion 5 to a mold cavity 7. The mold cavity may be of a shapecomprising a pipe portion 7a having an outer diameter of 60 mm, a lengthof 200 mm and a thickness of 2.5 mm, a flange portion 7b having an outerdiameter of 80 mm and a width of 3 mm, and a plurality of boss portions7c projecting out from the pipe portion 7a and having an outer diameterof 10 mm and a height of 20 mm. It should be noted that the shape of themold cavity 7 is not restricted to that described above. The innersurface of the mold cavity 7 is preferred to be coated with a moldcoating in a thickness of 0.01-0.4 mm, preferably 0.15 mm. On the upperend of the mold cavity 7, a riser 8a (also serves as a run-off) and agate 8b are provided. The vacuum vessel 2 and the cover 2a, the vacuumvessel 2 and the sand mold 4, and the cover 2a and the sand mold 4 arein contact with each other via packings 23 for maintaining the vacuumvessel 2 hermetically closed and preventing the vacuum degree of themold cavity 7 from decreasing.

The upper surface of the sand mold 4 facing the vacuum side is concavelycut out toward the riser 8a to form a suction recess 12. The bottom ofthe suction recess 12 is preferred to be close to the riser 8a (alsoserve as a run-off) unless the mold portion between the bottom of thesuction recess 12 and the riser 8a is broken due to a mechanical orthermal shock during the molding process. Specifically, the distancebetween the bottom of the suction recess 12 and the riser 8a ispreferred to be about 15-30 mm. The diameter of the suction recess 12 isnot specifically restricted unless the mechanical strength of the sandmold 4 is deteriorated, and may be determined based on the size of themold cavity 7, the riser 8a, etc. For example, the suction recess 12 mayhave a diameter of about 300 mm.

On the outer side of the vacuum vessel 2, a sensor 13 for detecting thatthe vacuum casting apparatus 1 is immersed into a melt 15 in a meltingfurnace 14 is provided.

The casting by the vacuum casting apparatus 1 shown in FIG. 1 isoperated by immersing the entrance portion 5 of the sand mold 4 into themelt 15 in the melting furnace 14. When the sensor 13 attached on theouter side of the vacuum vessel 2 detects the immersion of the entranceportion 5 into the melt 15, the downward moving of the vacuum vessel 2is ceased while the evacuation by the vacuum means 11 is initiated. Whenthe interior of the vacuum vessel 2 is evacuated, the air in the moldcavity 7 is also evacuated through the suction recess 12 which ensuresthat the mold cavity 7 is rapidly filled with the melt which flows intothe runner 6. The vacuum degree of the mold cavity 7 can be regulated bychanging the distance between the suction recess 12 and the riser 8a.

FIG. 2 is a schematic cross sectional view showing a modified embodimentof the vacuum casting apparatus shown in FIG. 1. The basic structure ofthe apparatus of FIG. 2 is the same as that of the apparatus of FIG. 1.Therefore, the same reference numerals are assigned to the memberscommon to FIGS. 1 and 2.

In the vacuum casting apparatus shown in FIG. 2, a porous member 16having a gas permeability larger than that of the mold 4 is disposedbetween the suction recess 12 and the riser 8a into which the melt 15 isfinally introduced. The porous member 16 is preferred to be formed byramming, for example, a molding sand coarser than the molding materialof the mold 4 into a disc, plate, etc. The porous member 16 may beformed as an integral part of the mold 4 or as an individual part.

It is necessary that the gas permeability of the porous member 16 islarger than that of the mold 4, and preferably, the former is about 3-30times the latter. For example, when a mold is formed of silica sand #6(gas permeability: 261) and a mold coating (gas permeability: 48), theporous member 16 is preferred to be formed of silica sand #5 (gaspermeability: 785) or silica sand #4 (gas permeability: 1130). The gaspermeability mentioned above was measured according to JIS Z 2603-1976(test method for gas permeability of molding sand).

The vacuum casting apparatus shown in FIG. 2 further has a partitionmember 19 composed of a impermeable material for separating the interiorof the vacuum vessel 2 into a mold chamber 17 and vacuum chamber 18. Bythe partition member 19, the evacuating force is concentrated in theparticular portion, in particular, the bottom of the suction recess 12which is opposite the last-filled portion of the mold cavity. Thepartition member 19 has an opening communicating with the suction recess12 and a projecting portion 19a extending downward to cover the side ofthe suction recess 12. A plate 20 having a central opening communicatingwith the suction recess 12 may be put on the top of the partition member19.

A grasping means 22 such as coil spring, etc. is disposed between theplate 20 and a flange 21 of the cover 2a projecting into the interior ofthe vacuum chamber 18. The elastic force of the grasping means 22 isexerted on the mold 4 through the plate 20 and the partition member 19thereby locating the mold 4 at a predetermined position in the moldchamber 17. A sealing member 23 such as packing, etc. is disposedbetween the plate 20 and the partition member 19 thereby airtightlyisolating the vacuum chamber 18 and the mold chamber 17.

The vacuum casting apparatus shown in FIG. 2 is further equipped with aprotecting frame 24 (made of steel, for example) which covers the sideof the entrance portion 5 and the bottom surface of the mold 4. Sincethe lower part of the protecting frame 24 projects downward from theopening 3 of the vacuum vessel 2, the protecting frame 24 is alsoimmersed into the melt 15 in the melting furnace 14 together with theentrance portion 5. The protecting frame 24 enhances the strength of theentrance portion 5, prevents the decrease in the vacuum degree of therunner 6, and further prevent the incorporation of air into the meltthrough the side of the entrance portion 5.

In the vacuum casting apparatus shown in FIG. 2, a supply means 25 isconnected to the vacuum vessel 2. The supply means 25 supplies an inertgas under pressure into the vacuum vessel 2 and replace the air in thevacuum vessel 4 with the inert gas. The preferred inert gas includesnitrogen gas, argon gas, etc.

The vacuum casting apparatus shown in FIG. 2 can be operated basicallyin the same manner as in the vacuum casting apparatus shown in FIG. 1.First, the atmosphere of the vacuum vessel 2 is replaced with an inertgas. To this end, the air in the vacuum vessel 2 is purged away bysupplying an inert gas from the supply means 25 to fill the vacuumvessel 2 with the inert gas. Then, the vacuum vessel 2 having the mold 4therein is moved downward to immerse the entrance portion 5 into themelt 15 in the melting furnace 14, followed by sucking the melt into therunner 6.

FIG. 3 is a schematic cross sectional view showing another modifiedembodiment of the vacuum casting apparatus shown in FIG. 1. The basicstructure of the apparatus of FIG. 3 is the same as that of theapparatus of FIGS. 1 and 2. Therefore, the description on the memberscommonly shown in FIGS. 1-3 is omitted here.

In this embodiment, a hollow core 26 is disposed within the mold cavity7. A hollow space 26a of the core 26 is communicated with the vacuumchamber 18 via a narrow suction duct 27 which communicates with thesuction recess 12. With this structure, the suction force can bedirectly exerted to the interior of the core 26. The mold 4 has narrowsuction ducts 28 which extend from the bottom of the suction recess 12to the vicinity of last-filled portions 8d and 8e of the mold cavity 7.The ducts 28 aid in rapid and entire filling of the core 26 and thelast-filled portions 8d and 8e with a melt. The vacuum casting apparatusshown in FIG. 3 can be operated in the same manner as in the vacuumcasting apparatus shown in FIG. 2.

3! Second embodiment

FIG. 4 is a schematic cross sectional view showing a vacuum castingapparatus according to a second embodiment of the present invention.

In this embodiment, the mold 4 has a runner 60 which extends, verticallyfor example, from the bottom of the entrance portion 5 to the vicinityof the suction recess 12 along at least partially the side of the moldcavity 7. The runner 60 communicates with the mold cavity 7 via threefilling passages 61a, 61b and 61c. Each of the passages 61a, 61b and 61cascends toward the mold cavity 7 so that the joining portion of thefilling passage and the mold cavity 7 is positioned upward the joiningportion of the filling passage and the runner 60. With this structure,the front of the melt flowing into the mold cavity 7 is scarcelydisturbed, and the cavity 7 can be rapidly filled with the melt.Incidentally, another runner directly communicating with the bottom ofthe mold cavity 7 may be provided, if necessary.

The vacuum casting apparatus shown in FIG. 4 can be operated in the samemanner as in the first embodiment except that the melt is rapidlyintroduced into the mold cavity 7 from the runner 60 extending along atleast partially the side of the mold cavity 7 via the filling passages61a, 61b and 61c. The vacuum degrees of the runner 60 and the moldcavity 7 are not necessarily the same. For example, at a certain stageduring the evacuation, it is preferable to set the pressure in therunner 60 about 50 mmHg lower than that of the mold cavity 7.

FIG. 5 is a schematic cross sectional view showing a modified embodimentof the vacuum casting apparatus shown in FIG. 4. The basic structure ofthe apparatus of FIG. 5 is the same as that of the apparatus of FIG. 4.Therefore, the description on the members commonly shown in FIG. 4 isomitted here.

In this embodiment, the mold 4 has a hollow core 62 incorporated intothe mold cavity 7. A hollow space 62a of the core 62 is communicatedwith the vacuum chamber 18 via a narrow suction duct 63 whichcommunicates with the suction recess 12. With this structure, thesuction force can be directly exerted to the interior of the core 62.The mold 4 also has a narrow suction duct 64 which extends from thebottom of the suction recess 12 to the vicinity of a last-filled portion65 of the mold cavity 7. The suction duct 64 aids in rapid and entirefilling of the cavity 7 with a melt. The vacuum casting apparatus shownin FIG. 5 can be operated in the same manner as in the vacuum castingapparatus shown in FIG. 4.

FIG. 6 is a schematic cross sectional view of a vacuum casting apparatuswhich has a fabricated mold (multi-cavity mold) consisting of aplurality of split molds and provided with a plurality of mold cavitiesso as to produce a plurality of cast articles in one casting operation.FIG. 7 is a cross sectional view of the apparatus of FIG. 6 taken alongthe A--A line. In FIGS. 6 and 7, although a four-cavity mold is shown, afabricated mold to be used in the present invention is not limited toit.

Each mold cavity 7 and riser 8a may be of the same shape as those shownin FIG. 4. Each of the mold cavities 7 communicates via three fillingpassages 61a, 61b and 61c with a common runner 60 extending along thevertical center line. The parting plane 90 is so made that the partingplane coincides with the vertical plane which includes the verticalcenter line passing through the runner 60 and divides each mold cavityinto tow partings. As seen from FIG. 7, the fabricated mold 91 isdivided into four split molds 92 of the same shape by two parting planes90 which perpendicularly intersect each other. In the same manner, ann-cavity mold may be fabricated from n split molds. By the use of thefabricated mold mentioned above, the cost for producing patterns, molds,etc. can be reduced. The vacuum casting apparatus in this embodiment maybe operated in the same manner as in the vacuum casting apparatus shownin FIG. 4.

The present invention will be described in more detail with reference tothe following examples. However, it is to be understood that theinvention is not intended to be limited to the specific embodiments.

EXAMPLE 1

A melt (1550° C.) having a composition shown in Table 1 was cast by thevacuum casting apparatus shown in FIGS. 1 and 2 to produce cast steelsof various thicknesses of at least 2.5 mm. Any casting defects such asinsufficient filling, etc. were not observed in the thin cast articles.

                  TABLE 1    ______________________________________    (weight %)    C      Si       Mn     Ni      Cr   Fe    ______________________________________    0.08   1.8      0.6    8.0     18.0 Balance    ______________________________________

EXAMPLE 2

A melt (1580° C.) having a composition shown in Table 1 was cast by thevacuum casting apparatus shown in FIG. 4 to produce cast steels ofvarious thicknesses of at least 2.0 mm. Any casting defects such asinsufficient filling, flow back, etc. were not observed in the thin castarticles.

EXAMPLE 3

A melt (1610° C.) having a composition shown in Table 1 was cast by thevacuum casting apparatus shown in FIG. 5 to produce cast steels ofvarious thicknesses of at least 1.5 mm. Any casting defects such asinsufficient filling, flow back, etc. were not observed in the thin castarticles.

EXAMPLE 4

In order to evaluate the flow of the melt in an apparatus having thestructure shown in FIG. 4, the flow of the melt in a mold for producinga manifold shown in FIG. 8 was observed and simulated by computer. Asshown in FIG. 8, the mold has a mold cavity 7 communicated with a runner60 via six filling passages 66a-66f. The results are shown in FIG. 8.The numerical values therein means the time (measured by second)required for the melt to reach the respective positions in the moldcavity.

As seen from FIG. 8, the melt drawn into the runner 60 was firstintroduced into the lower portion of the cavity 7 through the firstfilling passage 66a. Just before the level of the melt thus introducedreached the upper end of the second filling passage 66b, the meltpassing through the second filling passage 66b began to be introducedinto the mold cavity 7. Thereafter, just before the new level of themelt in the mold cavity 7 reached the upper end of the next fillingpassage, the melt passing through the next filling passage began to beintroduced into the mold cavity 7. This filling process was successivelyrepeated until the mold cavity 7 was entirely filled with the melt. Therising condition of the melt level is shown in FIG. 8 by broken lines.

Thus, since a melt with a little temperature lowering is poured on to amelt already introduced into the mold cavity, the casting defects suchas insufficient filling, leak defects, inclusion of air, blow holes,etc. can be effectively prevented.

The vacuum degrees of some portions of the vacuum casting apparatus,which may be employed to fill the mold cavity with the melt in a mannershown in FIG. 8, are shown in FIG. 9. As seen from FIG. 9, the fillingof the mold cavity 7 with the melt was completed within about onesecond. Further, it would be noted that in this period of time, thevacuum in the vacuum chamber 18 (suction recess 12) contributes toreducing the pressure in the runner 60 much more than to that of themold cavity 7. Namely, the vacuum degree of the runner 60 is higher thanthat of the mold cavity 7. In order to impart such a high vacuum degreeto the runner 60, the top end of the vertically extending runner 60 ispreferred to reach near the suction recess 12.

INDUSTRIAL APPLICABILITY

As described above, in the present invention, a suction recess isprovided at the vicinity of the mold cavity, riser or run-off includedin a mold, in particular, at the vicinity of the cavity portion where amelt is finally introduced (last-filled portion). With this suctionrecess, the suction effect of the melt into the cavity is enhanced, andalso the introduction of the melt into the last-filled portion can befacilitated. As a result, the casting defects such as insufficientfilling, etc. can be prevented. In addition, by disposing a porousmember having a gas permeability larger than that of a mold between asuction recess and a last-filled portion of a cavity, the vacuum degreesof the mold cavity, riser and run-off can be individually regulated,thereby enabling to control the flow speed of the melt.

Further, in the present invention, a runner is communicated with a moldcavity via a plurality of filling passages. With this structure, since amelt passing through one of the filling passages is introduced on to amelt already introduced in a mold cavity, the front temperature of themelt in the mold cavity can be prevented from lowering, thereby enablingto effectively avoiding insufficient filling, cold shut, shrinkagecavity, etc.

Since the vacuum casting apparatus and method of the present inventionhave technical advantages as described above, they are suitable forproducing remarkably thin cast articles of steel, in particular forproducing exhaust equipment members such as manifold, etc.

We claim:
 1. A vacuum casting apparatus comprising:(a) a vacuum vesselhaving at least one opening at the bottom thereof; (b) a mold disposedin said vacuum vessel and having a runner and a mold cavitycommunicating with said runner, said runner having an opening under saidopening of said vacuum vessel; and (c) a vacuum means communicating withsaid vacuum vessel; wherein a suction recess having an opening on thetop surface of said mold is disposed in a vicinity of a portion of saidmold cavity which is most distant from said opening of said runner andwhich is lastly filled with a melt of casting material, and wherein saidsuction recess is so disposed that a distance between the bottom of saidsuction recess and said portion of said mold cavity is smaller than adistance between the outer surface of said mold and any other portionsof said mold cavity, thereby rapidly filling said mold cavity with saidmelt.
 2. The vacuum casting apparatus according to claim 1, wherein aporous member having a gas permeability larger than that of said mold isdisposed between said suction recess and said mold cavity.
 3. The vacuumcasting apparatus according to claim 1 or 2, wherein a vacuum chamber isformed in a portion of said vacuum vessel communicating with said vacuummeans by covering with a partition member a mold surface facing saidvacuum chamber except for a mold surface defining the bottom of saidsuction recess.
 4. The vacuum casting apparatus according to claim 1,wherein said mold has a reverse-truncated cone projecting portion on thebottom surface thereof, said projecting portion projecting downward fromsaid opening of said vacuum vessel, having on its bottom surface saidopening of said runner, and the exposed surface of said projectingportion being covered with a protecting frame except for its bottomsurface.
 5. The vacuum casting apparatus according to claim 1, wherein apermeable, hollow core is disposed in said mold cavity, a hollow spaceof said hollow core being communicated with said suction recess via anarrow suction duct.
 6. The vacuum casting apparatus according to claim1, wherein said mold cavity is provided with a plurality of risers, andsaid mold is provided with at least one suction duct communicating withsaid suction recess and extending through said mold to a vicinity of oneof said risers other than one positioned in a vicinity of said suctionrecess.
 7. The vacuum casting apparatus according to claim 1, whereinsaid apparatus further comprising a supply means for supplying an inertgas into said vacuum vessel to replace the atmosphere of said vacuumvessel with said inert gas prior to evacuating said vacuum vessel.
 8. Avacuum casting method comprising the steps of:(a) disposing a moldhaving a runner and a mold cavity communicating with said runner in avacuum vessel having at least one opening at the bottom thereof so thatan opening of said runner is positioned under said opening of saidvacuum vessel; (b) disposing a suction recess having an opening on thetop surface of said mold in a vicinity of a portion of said mold cavitywhich is most distant from said opening of said runner and which islastly filled with a melt of casting material so that a distance betweenthe bottom of said suction recess and said portion of said mold cavityis smaller than a distance between the outer surface of said mold andany other portions of said mold cavity; and (c) evacuating said mold byoperating a vacuum means connected to said vacuum vessel thereby rapidlyfilling said mold cavity with said melt.
 9. The vacuum casting methodaccording to claim 8, wherein a porous member having a gas permeabilitylarger than that of said mold is disposed between said suction recessand said mold cavity, thereby filling said mold cavity with said meltmore rapidly.
 10. The vacuum casting method according to claim 8 or 9,wherein a vacuum chamber is formed in a portion of said vacuum vesselcommunicating with said vacuum means by covering with a partition membera mold surface facing said vacuum chamber except for a mold surfacedefining the bottom of said suction recess, thereby enhancing a suctioneffect of said suction recess.
 11. The vacuum casting method accordingto claim 8, wherein a permeable, hollow core is disposed in said moldcavity so that an open end of said hollow core is positioned in avicinity of said suction recess, thereby rapidly evacuating said moldcavity through said hollow core.
 12. The vacuum casting method accordingto claim 8, wherein said mold cavity is provided with a plurality ofrisers, and said mold is provided with at least one suction ductcommunicating with said suction recess and extending through said moldto a vicinity of one of said risers other than one positioned in avicinity of said suction recess, thereby evacuating said mold cavityalso through risers positioned in an area other than the vicinity ofsaid suction recess.
 13. The vacuum casting method according to claim 8,wherein said vacuum vessel is supplied with an inert gas to replace theatmosphere of said vacuum vessel with said inert gas prior to evacuatingsaid vacuum vessel.
 14. The vacuum casting method according to claim 8,wherein said vacuum vessel is evacuated after said opening of saidrunner is immersed into said melt maintained in a melting furnace.
 15. Avacuum casting apparatus comprising:(a) a vacuum vessel having at leastone opening at the bottom thereof; (b) a mold disposed within saidvacuum vessel and having a mold cavity formed therein, a runner havingan opening under said opening of said vacuum vessel and extending alongat least partially the side of said mold cavity, said mold cavitycommunicating with said runner through a plurality of filling passages,and a suction recess having an opening on the top surface of said moldand disposed in a vicinity of a portion of said mold cavity which ismost distant from said opening of said runner and which is lastly filledwith a melt of casting material; and (c) a vacuum means communicatingwith said vacuum vessel; whereby said mold cavity is evacuated throughsaid suction recess by operating said vacuum means more rapidly thanmold portions other than said mold cavity to rapidly fill said moldcavity with said melt.
 16. The vacuum casting apparatus according toclaim 15, wherein said plurality of filling passages are formed alongsaid runner and ascend toward said mold cavity, and the position andshape of each of said filling passages are defined so as to make arising level of a melt introduced into said mold cavity approximatelyequal in height to a level of a melt to be introduced from a nextfilling passage.
 17. The vacuum casting apparatus according to claim 15or 16, wherein said runner extends to a vicinity of said suction recessto enable said melt to rapidly rise in said runner as well as to fillsaid mold cavity with said melt.
 18. The vacuum casting apparatusaccording to claim 15, wherein a porous member having a gas permeabilitylarger than that of said mold is disposed between said suction recessand said mold cavity.
 19. The vacuum casting apparatus according toclaim 15, wherein a vacuum chamber is formed in a portion of said vacuumvessel communicating with said vacuum means by covering with a partitionmember a mold surface facing said vacuum chamber except for a moldsurface defining the bottom of said suction recess.
 20. The vacuumcasting apparatus according to claim 15, wherein said mold has areverse-truncated cone projecting portion on the bottom surface thereof,said projecting portion projecting downward from said opening of saidvacuum vessel, having on its bottom surface said opening of said runner,and the exposed surface of said projecting portion being covered with aprotecting frame except for its bottom surface.
 21. The vacuum castingapparatus according to claim 15, wherein a permeable, hollow core isdisposed in said mold cavity, a hollow space of said hollow core beingcommunicated with said suction recess via a narrow suction duct.
 22. Avacuum casting method comprising the steps of:(a) disposing in a vacuumvessel having at least one opening at the bottom thereof a mold having amold cavity formed therein and a runner extending along at leastpartially the side of said mold cavity, said mold cavity communicatingwith said runner through a plurality of filling passages, so that anopening of said runner is positioned under said opening of said vacuumvessel; (b) disposing a suction recess having an opening on the topsurface of said mold in a vicinity of a portion of said mold cavitywhich is most distant from said opening of said runner and which islastly filled with a melt of casting material so that a distance betweenthe bottom of said suction recess and said portion of said mold cavityis smaller than a distance between the outer surface of said mold andany other portions of said mold cavity; and (c) evacuating said mold byoperating a vacuum means connected to said vacuum vessel thereby rapidlyfilling said mold cavity with said melt.
 23. The vacuum casting methodaccording to claim 22, wherein a porous member having a gas permeabilitylarger than that of said mold is disposed between said suction recessand said mold cavity, thereby filling said mold cavity with said meltmore rapidly.
 24. The vacuum casting method according to claim 22 or 23,wherein a vacuum chamber is formed in a portion of said vacuum vesselcommunicating with said vacuum means by covering with a partition membera mold surface facing said vacuum chamber except for a mold surfacedefining the bottom of said suction recess, thereby enhancing a suctioneffect of said suction recess.
 25. The vacuum casting method accordingto claim 22, wherein a permeable, hollow core is disposed in said moldcavity so that an open end of said hollow core is positioned in avicinity of said suction recess, thereby rapidly evacuating said moldcavity through said hollow core.
 26. The vacuum casting method accordingto claim 22, wherein said mold cavity is provided with a plurality ofrisers, and said mold is provided with at least one suction ductcommunicating with said suction recess and extending through said moldto a vicinity of one of said risers other than one positioned in avicinity of said suction recess, thereby evacuating said mold cavityalso through risers positioned in an area other than the vicinity ofsaid suction recess.
 27. The vacuum casting method according to claim22, wherein said vacuum vessel is evacuated after said opening of saidrunner is immersed into said melt maintained in a melting furnace.